Abstract
By bioassay-guided fractionations, quercetin was isolated as the principal tyrosinase inhibitor from the dried flowers of Heterotheca inuloides Cass (Compositae). This flavonol inhibited the oxidation of L-3,4-dihydroxyphenylalanine (L-DOPA) by mushroom tyrosinase (EC 1.14.18.1) with an ID 50 of 22 μg/ml (0.07 mM). Interestingly, quercetin 3-β-glucoside did not inhibit this oxidation up to 1 mg/ml. Kinetic study have found quercetin to be a characteristic competitive inhibitor for the L-DOPA oxidation by the mushroom tyrosinase. Based on the above findings, several related flavonoids were also examined for comparison.
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